Radio Frequency Identification (RFID) tags used in chemical or biological sensors, such as RFID tag 10 illustrated in FIG. 1, are known. Such RFID tags 10 detect signals by measuring a single parameter. RFID tag 10 includes a substrate 12 upon which are positioned an antenna 14 and a capacitor 16. As used in this patent application, an antenna is defined as a circuit that includes a resistor element, an inductance element and a capacitor element. Among the disadvantages of RFID tags 10 as chemical and/or biological sensors are difficulties in performing chemical or biological measurements in the presence of chemical and physical interferences.
Some known chemical or biological sensors include attaching an identification marker with an antibody. For example, some chemical/biological sensors include a chip attached to an antibody, wherein the chip includes a fluorescent marker identifying the specific antibody.
Some known chemical or biological sensors include structural elements that are formed from a material that selectively responds to a specific analyte. See, U.S. Pat. No. 6,359,444. Other known chemical or biological sensors include an electromagnetically active material that is located in a specific position on the sensors and that may be altered by an external condition. See, U.S. Pat. No. 6,025,725. Some known chemical or biological sensor systems include components for measuring more than one electrical parameter. See, U.S. Pat. No. 6,586,946.
In practical situations, any wireless chemical or biological sensor may not be accurately positioned with respect to a receiver antenna. Thus, the response of the wireless sensor will be affected by such position differences. The sensitivity of the sensor response (defined as a level of the sensor signal change upon exposure to a certain concentration of analyte) is dependent on the antenna-to-sensor position.